Equipment for the motorization of garage doors or entries with radiofrequency control, as described in U.S. Pat. No. 4,750,118, generally contains a push-button used to set the command receiver into a learning mode. Once it has been placed in the learning mode, the command receiver can thus record the identifier of the first command transmitter to be actuated, which then becomes valid for controlling the actions of the element operated by the command receiver. The security of this learning mode derives from the fact that the equipment is placed inside the property, and that only the property owner is supposed to be able to access the push-button. It is clear that it is necessary to shorten the duration of this learning mode in order to avoid learning a remote transmitter transmitting in the same time slot. The ideal situation would be to require continuous pushing on the button of the command receiver while pressing on a key of the command transmitter to be paired up, but this method is scarcely convenient. For reasons of cost and ergonomics, it may therefore be desirable to do without such a push-button belonging to the command receiver, and to directly use the mobile remote-control constituting the command transmitter. In this particular learning mode, it will then be necessary to ensure that the two elements are only a few centimeters or tens of centimeters apart.
As described in EP 0 921 507, it is known to reduce the sensitivity of the receivers. It is clear, however, that such a receiver with reduced sensitivity may be illegitimately operated by a very high power transmitter.
In a related field, the gates for locking industrial or commercial closures require an operation of the “deadman's” type, commonly produced by using a monostable key switch. Providing this switch in proximity to the gate guarantees that the user can see the device as it moves, in order to avoid any accident. For reasons of vandalism and convenience, it would be desirable to be able to replace this lock-and-key assembly by a simple remote-control (which can furthermore be used for other applications such as controlling the lighting and solar protection). In the “deadman's” mode, it is therefore essential for this remote-control to work only in proximity to the command receiver, which may be arranged in the masonry or behind a glazing so as to prevent any vandalism.
There is therefore a need for a method and means which guarantee that a command transmitter of the radio-electrical type, the normal range of which is between about ten and several hundred meters, to be located in the immediate proximity of the command receiver in order to validate the transmission of particular commands. This method and these means should under no circumstances be illegitimately operated by a very high power remote transmitter.
Differential measurement carried out on two antennas placed a distance apart is well known to the person skilled in the art for determining the direction of a far-field transmitter according to the methods of radiogoniometry. In a conventional installation, two generally perpendicular coils may be used which can be oriented by rotation about their common diameter. The direction of the transmission is found when the received signals are equal. This direction may also be determined from the ratio of the intensities received by each coil, if these are fixed.
U.S. Pat. No. 3,553,699 describes an instrument for detecting the direction of a radio source, in which these two perpendicular fixed antennas are combined with Hall effect sensors that are used as modulators.
It will be noted that these installations required two perpendicular coils, and that they are not used either for measuring distance or for detecting proximity of the source.
For the field of angular determination, it is possible to provide antennas which this time are placed a distance apart and in the same plane, but by using direct phase-shift measurements on the received signals, as described in U.S. Pat. No. 3,697,997, or by alternatively by cross-correlation as in U.S. Pat. No. 4,876,549.
These fairly complex methods cannot in any event be utilized for measuring distance or for detecting proximity of the source, since a phase-shift measurement can only be used to within an integer number of wavelengths.
For applications relating to access control, it has nevertheless been envisaged to provide a plurality of communication modes between a mobile transponder and a fixed receiver. For instance, U.S. Pat. No. 5,552,641 describes an automobile security system. It will be noted that the system contains two antennas that can be used in reception and can be arranged in a common plane when, for example, they are placed in the external rear-view mirrors of the vehicle. But each antenna is individually connected to at least one HF or LF radio receiver. The intended object is, in particular, to provide sufficient redundancy to guarantee good transmission on at least one of the communication circuits, between the base fixed in the automobile and the mobile transponder. According to that invention, it is possible to determine which door to open according to the receiver which is activated.
For the same type of application, U.S. Pat. No. 5,751,073 describes an RFA activation module which may include two antennas, one of which is intended for detecting the transmitter outside, while the other is intended for detecting it inside. The transmitter may be a transponder.
Lastly, U.S. Pat. No. 6,087,987 describes a method for locating a validation component (transponder) inside an automobile passenger compartment. This method is based on measuring the amplitudes or intensities of fields which are individually measured by at least two transmitter-receivers arranged in the passenger compartment, each of these transmitter-receivers therefore being provided with a device for measuring intensity.
It will be noted that both of these patents use the fact that a “remote” link to a transporter has a short range, for example limited to one meter. No use is made of any near-field relations. For example, column 4, lines 53-59 of U.S. Pat. No. 6,087,987 states that the received power decreases quadratically with the distance. Such a decrease as 1/r2 (where r is the distance from the transmitter to the receiver) is characteristic of far-field conditions, while the decrease is as 1/r in the near field, as is known to the person skilled in the art and is mentioned, for example, in “Reference Data for Engineers—Ninth Edition—Marc E. Van Valkenburg, Wendy M. Middleton—Newnes”, pages 32-7.
The device described in U.S. Pat. No. 6,087,987 can very well be illegitimately operated by a bogus transponder which would contain both a highly sensitive receiver and a very high power transmitter and would be located outside the vehicle, even at tens of meters from it. This is because, ignoring the effects of attenuation due to the metal car body, each of the receivers will then receive a signal of virtually equal power and the system will logically conclude that there is a transponder in the middle of the passenger compartment.
In the field of theft detection, or more generally remote electronic identification (RFID), it has been envisaged to use discrimination characteristics associated with the field characteristics involving constructive or destructive interference between a plurality of antennas which constitute an array. For instance, U.S. Pat. No. 4,016,553 describes a device in which at least two transmission coils, contained in two parallel planes, are serially connected in a crossed fashion. The dimensions of the coils and the spacing between them are very small here relative to the wavelength (for example a tenth of the wavelength). In the case of a far-field distance (a few meters), there are therefore destructive interferences between the waves transmitted by the two coils. In other words, the “interrogation zone” is limited to the vicinity of the transmission system (circle ET in FIG. 1) which avoids radiating the electromagnetic energy in a large perimeter and makes it possible to comply with the regulations governing this. In the same way, an identical device is used for reception. Therefore, the presence of an element that perturbs the field will be picked up inside the circle ER. Any perturbing transmission that comes from far-field points will not be detected by this arrangement.
Patent Application DE 101 16 870 discloses a device comprising a command transmitter and a command receiver. The receiver comprises means for locating the transmitter by measuring the level of the signals constituting the commands which it receives, and it interprets these commands as a function of the location of the transmitter with respect to the receiver.
U.S. Pat. No. 5,170,172 discloses a device for indicating the distance between a transmitter and a receiver of radio-electrical signals. The receiver comprises a plurality of radio antennas which are preferably arranged mutually orthogonally in order to make it possible to obtain a good image of the power of the radio-electrical signal, irrespective of the orientation of the receiver.
In the field of meteorology, U.S. Pat. No. 3,715,660 describes a device intended to measure the distance separating it from the lightning strikes during a thunderstorm. The device comprises an antenna of the coil type for measuring the magnetic component of the electromagnetic wave produced by the lightning strike, and an antenna of the quarter-wave type for measuring the electric component of the wave. Analysis of the ratio between these two quantities makes it possible to determine the distance to the source of the magnetic wave.
The devices described above do not make it possible to implement a method which guarantees that a command transmitter of the radio-electrical type is located in the immediate proximity of the command receiver, in order to validate transmission of commands, and which guarantees that the device is safe against a very high power transmitter.
It is an object of the invention to provide a device for implementing a method which overcomes this drawback and which improves the methods known in the prior art. In particular, the invention proposes to provide a device for carrying out a method which guarantees that a command transmitter of the radio-electrical type is located in the immediate proximity of the command receiver, in order to validate transmission of commands, and which guarantees that the device is safe against a very high power transmitter.
The concept of a so-called near-field zone and a so-called far-field zone is defined by using the wavelength λ of the radio-electrical signal, on the basis of the characteristics of the electromagnetic wave. For antennas whose size is small compared with the wavelength, the transition distance between “near field” and “far field” is λ/2π (cited ref. pages 32-4). A signal transmitted from a point lying more than ⅙ of the signal wavelength away from a receiver is then said to be transmitted from a far-field zone. A signal transmitted from a point lying less than ⅙ of the signal wavelength away from a receiver is said to be transmitted from a near-field zone. In the vicinity of the receiver, the electromagnetic field due to a signal is referred to as far-field if it was transmitted from a far-field zone. In the vicinity of the receiver, the electromagnetic field due to a signal is referred to as near-field if it was transmitted from a near-field zone. For a signal having a frequency of 433 MHz, the transition between near field and far field takes place about 12 cm away from the transmission point.
These theoretical distances therefore depend on the type of antenna. What is important is that a distance can be associated with an electromagnetic characteristic of the field.